Dielectric block bandpass filters, for example as in U.S. Pat. No. 4,431,977, "Ceramic Bandpass Filter", are commonly used as signal filters in communication systems, for example as in a conventional radio, transceiver, or radiotelephone. Conventional dielectric filters offer advantages in both physical and electrical performance which make them ideally suited for use in mobile and portable radio transceivers. Multi-resonator dielectric filters, as depicted in FIG. 1, typically comprise a plurality of nearly quarter-wavelength transmission line resonators, constructed by making through-holes in the dielectric material, and plating these holes with a conductive material. In such a configuration, reactive coupling between adjacent resonators can be controlled by the physical dimensions of each resonator and by the orientation of each resonator with respect to the other resonators.
It is also commonplace to use diodes for electronically switching components in applications such as voltage controlled oscillators, duplexers, and other areas where high speed switching is required. Voltage variable capacitors are also used for tuning in applications such as electronically tuned helical resonators, for example in U.S. Pat. No. 4,459,571, "Varactor-tuned Helical Resonator Filter".
Altering the frequency characteristics of resonant loads is not a new idea, and such a resonant load may be a VCO, as in U.S. patent application Ser. No. 538,874, "Diode Bias Networks for Use With Voltage Controlled Oscillators", filed on behalf of Gehrke et al. on June 15, 1990, and assigned to the assignee of the present invention. Unfortunately, such a configuration relies on relatively high voltage levels generated by the VCO in order to cause self-rectification in the diode switch, resulting in a reverse bias condition. In typical filter applications, there are not sufficient voltage levels to guarantee reverse biasing, hence the switch could never be in an `off` state using this technique. Regarding the use of voltage variable capacitors, these devices typically offer very poor temperature stability, thereby requiring additional components for temperature compensation. The cost of adding these components can be significant when compared to the total cost of the filter.
In short, known solutions to the bandwidth adjustment of filters used in communication systems, especially for miniaturized, dielectrically loaded resonator filters, are inadequate when a real-time adjustable bandwidth is desired. It would be advantageous to have one filter which could be electronically configured for use in multiple frequency environments, such as the case in a conventional cellular radiotelephone operating domestically in one frequency environment and in a different frequency environment in a foreign country. Clearly, there is a need for a temperature stable, electronically-selectable-bandwidth filter which is not constrained by the aforementioned shortcomings.